This invention pertains to the input signal for the primary windings in a linear variable differential transformer hereafter LVDT or a synchronous resolver hereafter SR. Such devices are readily available and are disclosed in the early prior art, see, for example, the Macgeorge U.S. Pat. No. 2,427,866 covering an LVDT wound with symetrically spaced identical secondaries adjacent to a central primary on a common coil form wherein the core is longer than the primary winding. Similarly, a synchronous resolver is shown in the Schaevitz U.S. Pat. No. 2,494,493 wherein the coil form contains a pair of identical windings, and spaced symmetrically above and below the middle and within the coil is a core being a cardioidal shaped magnetic pivotally and eccentrically mounted on an input shaft positioned diametrically across the coil.
Linear movement of the core (LVDT) or rotation of the shaft (SR) influences the phase and amplitude of the waveform generated in the secondary in relation to the amount of movement or rotation.
The cross-section of a typical LVDT consists of three symmetrically spaced coils a primary and a pair of secondaries connected in series carefully wound about an insulated bobbin and four wire leads exited through one end. An outer shield of a ferro-magnetic material is placed over the windings which are vacuum impregnated with a suitable potting compound. Consequently, the finished transformer is impervious to humidity and magnetic influences. The core is made of a uniformly dense cylinder of nickel-iron alloy which is annealed to improve its homogeniety with respect to magnetic permeability. The LVDT is a frictionless device since there is no physical contact between the movable core and the LVDT coil structure. The absence of friction and contact between the coil and the core of an LVDT means that there is nothing to wear out and gives the LVDT an essential infinite mechanical life.
The nominal linear range of travel of an LVDT is the distance the core may be displaced in either direction from its null position. The symmetry of the LVDT construction provides null point repeatability. More particularly, the LVDT produce an electronic output proportional to the placement of the movable core. A waveform excitation applied to the primary induces a similar excitation in the secondary. The two identical secondaries are symmetrically spaced from the primary and adjacent thereto in axial relation therewith on each side thereof. The secondaries are connected in a series opposing circuit. As the motion of the noncontacting magnetic core varies, the magnetic inductance of each secondary relative to the primary is thereby determined by the induced voltage difference. If the core is moved off center, the magnetic inductance of the primary with respect to one secondary will be greater than with respect to the other and a differential voltage will appear across the secondary output terminals. For offset displacements within the normal operating range, the voltage is a linear function with respect to displacement with some deviation due to tolerances in fabrication of the LVDT.
When the middle of the core is centered between the secondary windings, i.e., is at the center point of the primary winding, the voltage induced in each secondary is equal and 180.degree. out of phase so there is no secondary output.
Certainty of waveform shape and frequency is absolutely essential to proper measurement of small differences in linear displacement at high periodicities. Therefore, uniform waveforms with a frequency greater than that of the oscillation of the core are required for accuracy and repeatability. The use of 60 Hz power line frequency for excitation of the primary coil is acceptable for core oscillations under six cycles per second. The rule of thumb is that the excitation frequency must be at least ten times greater than the highest modulation frequency to be measured as a component of mechanical motion.
The majority of LVDT applications apply sine waveforms to the primary coil which waveforms should be free from harmonic distortions since modulated distortion may increase the null voltage. Excessive null voltage requires filtering the excitation voltage and/or the LVDT output to remove harmonics which affect the accuracy in connection with measuring small displacements at high frequencies. Moreover, the variations in output from one LVDT to another require a means by which the LVDT's can be equalized and their deviation from linearity can be minimized to a point where it is negligible.
The improvement of the present invention will provide a uniform high frequency waveform and a means to specify the point on said waveform where the amplitude of the output from the secondaries will be used. Similarly, the output from the secondaries of any given LVDT will be corrected to overcome any deviations due to particular characteristics of the LVDT relative to an ideal LVDT with totally linear response.